1. Field of the Invention
The present invention relates to handheld tools for machining of workpieces. More specifically, the present invention is directed to such handheld tools as used to perform any one or more of drilling, milling, and abrading operations, especially for precise circuitry and substrate repair, as well as for modification and conformal coating removal, with respect to printed circuit boards.
2. Description of Related Art
During printed circuit board assembly or repair, occasions arise which require extra holes to be drilled, coatings or solder masks to be ground away, conductive traces to be severed, buried conductors to be located or excavated, card edge contacts to be burnished or polished, or other machining operations to be performed. In the past, a technician would use a tool such as a simple electric hand drill. Examples of hand drills with bits for performing such circuit board machining operations are the MAXI DRILL and MINI DRILL by Sterling (a Wahl Clipper corporation), the Weller model 601 and the Dremel model 232 tools. These types of tools may have been acceptable with older technology printed circuit boards, but they pose numerous problems for newer, densely populated, multilayer boards.
For example, the electronics industry is now very conscious of touching a tool of unknown electric potential to a circuit board populated with expensive, sensitive semiconductors since electrical overstress (EOS) damage can result, such as from transient voltages of the tool's electric motor being coupled to the tip and transferred to the circuit board. Additionally, tightly packed assemblies require precision since it is difficult to target a point of interest without touching adjacent components and conductors. Existing products lack adequate speed control to prevent abrupt starting of the tool which can cause jerking, jumping or skipping of the tool off of the target and fast starting speeds also lead to "wandering" of the tool. The operator often tenses up, squeezing hard to limit instability, and this leads to operator fatigue within a short time. Abrupt starting can also cause torques to be applied which are, themselves, damaging to sensitive/delicate workpieces. Not only is fast starting a problem, but so is the inability of such known tools to obtain a virtually immediate stopping of the tool bit when the operator releases the control switch. That is, when the control switch is released, the tool coasts to a stop and while coasting, as the operator relaxes or as the tool bit is extracted from the work, e.g., a drilled hole may become out-of-round, or a conductor (which on some boards can be quite thin) can be damaged, especially when drilling down to interior layers of a multilayer board (where it is also possible for a conductor to be severed if the operator inadvertently fails to release the control switch in time).
Outside of the circuit board environment, handheld drills are known which have a torque limiting feature (see, U.S. Pat. No. 4,487,270), as are handheld drills which have a rotational speed control (see, U.S. Pat. No. 4,292,571). However, these controls are for protection of the tool not the workpiece.
Additionally, a handheld drill is known (see, U.S. Pat. No. 4,650,375) which has a dynamic braking system for stopping a drill bit when it contacts an electrically conductive material. More specifically, to prevent electrocution of a person accidentally drilling into an electric power line, as well as interruption of the power supply should the power line be severed, a comparator/detector control is connected to the drill bit and motor circuit. With this control, upon detecting ground or power, an operational amplifier becomes unbalanced, resulting in a detection output signal being produced which, in turn, leads to the circuit to the motor windings being momentarily opened, after which the motor windings are briefly reversed and then disabled. However, the control of this drill brake requires high excitation voltages of the type found in 120 volt power lines, and cannot be made sensitive enough to respond to voltages in the millivolt range (which are required for safe use with printed circuits and sensitive electrical components found on circuit boards) without the control circuit being triggered by leakage voltages from the drill motor circuit.
Of course, drill presses with computer-controlled control systems are known (see, U.S. Pat. No. 4,745,557) which enables multi-ply workpieces of different thicknesses and hardnesses to be worked upon. In the particular control system of U.S. Pat. No. 4,745,557, the translational and rotational velocities of the drill bit can be continuously controlled using a feedback system which receives information from an optical beam sensor. However, such a system is not practical for a handheld tool, and even if it were, the various problems associated with printed circuit board machining operations, mentioned above, would still not be addressed.